In the process of electrophotographic printing, a charge-retentive surface, also known as a photoreceptor, is charged to a substantially uniform potential, so as to sensitize the surface of the photoreceptor. The charged portion of the photoconductive surface is exposed to a light image of an original document being reproduced, or else a scanned laser image created by the action of digital image data acting on a laser source. The scanning or exposing step records an electrostatic latent image on the photoreceptor corresponding to the informational areas in the document to be printed or copied. After the latent image is recorded on the photoreceptor, the latent image is developed by causing toner particles to adhere electrostatically to the charged areas forming the latent image. This developed image on the photoreceptor is subsequently transferred to a sheet on which the desired image is to be printed. Finally, the toner on the sheet is heated to permanently fuse the toner image to the sheet.
One familiar type of development of an electrostatic image is called “two-component development”. Two-component developer material largely comprises toner particles interspersed with carrier particles. The carrier particles are magnetically attractable, and the toner particles are caused to adhere triboelectrically to the carrier particles. This two-component developer can be conveyed, by means such as a “magnetic roll,” to the electrostatic latent image, where toner particles become detached from the carrier particles and adhere to the electrostatic latent image.
In magnetic roll development systems, the carrier particles with the triboelectrically adhered toner particles are transported by the magnetic rolls through a development zone. The development zone is the area between the outside surface of a magnetic roll and the photoreceptor surface on which a latent image has been formed. Because the carrier particles are attracted to the magnetic roll, some of the toner particles are interposed between a carrier particle and the latent image on the photoreceptor. These toner particles are attracted to the latent image and transfer from the carrier particles to the latent image. The carrier particles are removed from the development zone as they continue to follow the rotating surface of the magnetic roll. The carrier particles then fall from the magnetic roll and return to the developer supply where they attract more toner particles and are reused in the development process. The carrier particles fall from the magnetic roll under the effects of gravity or are directed away from the roller surface by a magnetic field.
Different types of carrier particles have been used in efforts to improve the development of toner from two-component developer with magnetic roll development systems. One type of carrier particle is a very electrically insulated carrier and development systems using developer having these carrier particles typically develop lines and fine detail with high fidelity. Development efficiency for solid areas, however, is increased through low magnetic field agitation in the development zone along with close spacing to the latent image and elongation of the development zone. The magnetic field agitation helps prevent electric field collapse caused by toner countercharge in the development zone. The close spacing increases the effective electric field for a potential difference and the longer development zone provides more time for toner development. A disadvantage of this type of development system is the tendency for the carrier beads to retain countercharge left by toner particles that were developed from the brush. Retention of the countercharge causes carrier beads to be lost to the photoreceptor background areas. This loss is undesirable and leads to contamination problems in the xerographic system as well as depletion of the developer sump over time. Other two-component developers have used permanently magnetized carrier particles because these carrier particles dissipate toner countercharge more quickly by enabling a very dynamic mixing region to form on the magnetic roll.
Another type of carrier particle used in two-component developers is an electrically conductive carrier particle. Developers using this type of carrier particle are capable of being used in magnetic roll systems that produce toner bearing substrates at speeds of up to approximately 100 pages per minute (ppm). These developers typically recruit toner for the latent electrostatic image from areas near the tip of the developer magnetic brush that are proximate the surface of the photoconductor because the electric fields are high in this region. The electrical conductivity of the carrier particles serves to prevent development field collapse caused by the retention of toner countercharge and thereby allows high efficiency development, especially of solid area latent images. This type of developer, however, supplies an adequate amount of toner for high speed xerography with difficulty because the only toner available for development is the toner near the tip of the magnetic brush. Consequently, high development roller speeds are required. Unfortunately, high roller speeds increase the wear on the rollers and decrease the life of the rollers. Another problem that occurs with this type of developer is the tendency of the carrier particles, when the toner concentrations are low, to charge up in the image electric field. This charge causes the carrier particles to develop onto the image areas of the photoreceptor and leads to white spot deletions in the final image as well as carrier bead contamination in the system.
Another type of carrier particle used in two-component developers is the semiconductive carrier particle. Developers using this type of carrier particle are also capable of being used in magnetic roll systems that produce toner bearing substrates at speeds of up to approximately 100 pages per minute (ppm). Developers having semiconductive carrier particles use a relatively thin layer of developer on the magnetic roll in the development zone. This feature allows more of the toner to be recruited during development than thick brush conductive developers allow. In these systems an AC electric waveform is applied to the magnetic roller to cause the developer to become electrically conductive during the development process. The electrically conductive developer increases the efficiency of development by preventing development field collapse due to countercharge left in the magnetic brush by the developed toner. A typical waveform applied to these systems is, for example, a square wave at a peak to peak amplitude of 1000 Volts and a frequency of 9 KHz. This waveform controls both the toner movement and the electric fields in the development zone. Typically these systems run in a “with” mode, which means the magnetic roll surface runs in the same direction as the photoreceptor surface. This movement in the same direction tends to keep background development low, but it has been observed to produce inadequate development unless high magnetic roller surface speeds are used to get an adequate supply of toner into the development zone. This high magnetic roll surface speed requires high strength magnets to control the developer bed. These types of magnets are expensive. Additionally, high speeds also increase the wear on bearings in the developer housing.
Another limitation of known magnetic roll systems used with developers having semiconductive carrier particles is the difficulty in extending the development zone to increase the time in which toner development may occur. One method for increasing development zone length with other developers having insulated or conductive carrier particles is to use two magnetic rolls. The two rolls are placed close together with their centers aligned to form a line that is parallel to the photoreceptor. Because the developer layer for semiconductive carrier particle developer is so thin, magnetic fields sufficiently strong enough to cause semiconductive carrier particles to migrate in adequate quantities from one magnetic roll to the other magnetic roll also interfere with the transfer of toner from the carrier particles in the development zones. Consequently, construction of the magnetic rolls requires careful consideration of this interference. If two rolls are not able to be used to increase the development zone, then the radius of the magnetic roll may be increased to accommodate this goal. There is a limit, however, to the diameter of the magnetic roll. One limit is simply the area within the printing machine that is available for a development subsystem. Another limit is the size and strength of the magnets internal to the magnetic roll that are required to provide adequate magnetic field strengths and shapes at the surface of a larger magnetic roll. Another problem with semiconductive development systems is a defect in which the system has trouble developing a halftone adjacent and following a solid so a halo of the solid is left at the boundary of the halftone. This happens at high toner concentrations and limits the latitude of the system.
The systems and methods discussed below address the limitations of development subsystems using developer having semiconductive carrier particles that have been noted.